Interplane magnetic coupling effects in the multilattice compound Y2Ba4Cu7O15

We investigate the interplane magnetic coupling of the multilattice compound Y₂Ba₄Cu₇O₁₅ by means of a bilayer Hubbard model with inequivalent planes. We evaluate the spin response, effective interaction, and the intra- and interplane spin-spin relaxation times within the fluctuation exchange approximation. We show that strong in-plane antiferromagnetic fluctuations are responsible for a magnetic coupling between the planes, which in turns leads to a tendency of the fluctuation in the two planes to equalize. This equalization effect grows with increasing in-plane antiferromagnetic fluctuations, i.e., with decreasing temperature and decreasing doping, while it is completely absent when the in-layer correlation length becomes of the order of one lattice spacing. Our results provide a good qualitative description of NMR and NQR experiments in Y₂Ba₄Cu₇O₁₅.